During the construction of underground wells, it is common, during and after drilling, to place a liner or casing, secured by cement pumped into the annulus between the wellbore and the outside of the liner. The cement serves to support the liner and to provide isolation of the various fluid-producing zones through which the well passes. In order to fulfill this function, it is necessary that the cement be present as an impermeable continuous sheath. However, for various reasons, over time the sheath placed in the annular space between the casing and the formation can lose its integrity (for example, deteriorate and become permeable) and hence its ability to maintain zonal isolation.
This loss of integrity is associated with formation of cracks or micro annulus in the cement matrix and can result from stresses imposed on the set cement due to cyclic pressure or thermal expansion and contraction of the casing, repeated impacts from the drill bit, chemical erosion from aggressive formation fluids/gases and external forces on the cement. The deterioration can be due to physical stresses caused by tectonic movements, temperature effects, chemical degradation of the cement, or various other reasons.
When cracks or other failures of the cement sheath do occur, it is common to attempt to pump and place one or more settable fluids or slurries into the failed part of the sheath in an attempt to regain isolation or well integrity. This procedure is generically referred to a remedial cementing.
There have been a number of proposals to deal with the problems of deterioration of the cement sheath over time. For example, an early approach was to make the set cement more flexible by addition of fibres or by making foamed cement. Although these methods can help to minimize formation of cracks or micro-annuli they do not eliminate formation of cracks or micro-annuli.
Another approach has been to develop and use self-adaptive cement/self-healing cement to solve the problem of crack or micro annulus formation and loss in zonal isolation. These cements contain a product that will swell on exposure to one or more types of downhole fluids and effectively block/seal cracks or micro annulus.
The following publications illustrate the use of self-adaptive/self-healing cement formulations containing a chemical that swells when exposed to liquid or gaseous hydrocarbons: US 2007/0137528 A1, US 2009/0139710 A1, U.S. Pat. Nos. 7,607,482, 7,607,484, 7,617,870, 7,647,970, 8,030,253, SPE/IADC 105781—Cavanagh et al., Self-Healing Cement—Novel Technology to Achieve Leak-Free Wells, SPE 110523—Moroni et al., Overcoming the Weak Link in Cemented Hydraulic Isolation, IADC/SPE 128226—Le Roy-Delage et al., Self-Healing Cement System—A Step Forward in reducing Long-Term Environmental Impact. Self-adaptive cement system that can compensate for changes or faults in the physical structure of cement after setting WO/2004/101951.
The following publications illustrate the use of self-adaptive/self-healing cement formulations containing a chemical that swells when exposed to methane: European Patent Application EP 2 450 417, European Patent Application EP 2 487 141, European Patent Application EP 2 518 034.
PCT Publication No. WO 2004/101925 illustrates the use of self-adaptive/self-healing formulations containing a chemical that swells when exposed to water in oil and gas wells.
European Patent Application EP 2 199 539 A1 illustrates the use of self-adaptive/self-healing cement formulations containing a combination of two chemicals: one that swells when exposed to liquid and/or gaseous hydrocarbons; and a second that swells on exposure to water oil and gas wells.
European Patent Application EP 2 404 884 A1 illustrates the use of self-adaptive/self-healing formulations containing a chemical that swells when exposed to carbon dioxide.
The following references illustrate the use of self-adaptive/self-healing cement formulations containing a chemical that swells when exposed to liquid and/or gaseous hydrocarbons and/or water and/or carbon dioxide: European Patent Application EP 2 025 732 A1, European Patent Application EP 2 025 732 A1.
U.S. Pat. No. 7,530,396 illustrates the use a self-adaptive/self-healing cement formulation containing an elastomeric material that exhibits a phase transition and/or cold flow behavior at bottom hole static temperature.
The following publications illustrate the use of cement formulations containing a significant amount of cement that is left intentionally un-hydrated when the cement sets and hydrates only when or if the cement sheath is damaged: WO 2008/034461 A1 and US Patent Application 2010/0081733 A1.
U.S. Pat. No. 8,236,100 discloses methods of characterizing the self-healing properties of a set cement in contact with hydrocarbons in an oil and/or gas well.
Methodologies for enhancing the compressive strength of cement-based materials, or for remediating/repairing these materials, based on the application of microbes, have been developed in recent years. One such methodology uses mineral-producing bacteria, which are capable of precipitating calcium carbonate (CaCO3). Mineral-producing bacteria have been used for the consolidation of sand columns, healing of cracks in granite, for surface treatment of limestone, including for healing creaks in cement-based materials. See for example, Jonkers, H. M et al. (2010) Ecological Engineering 36: 230-235; Van Tittelboom et al. (2010) Cement and Concrete Research 40: 157-166; Arnold, D. (2011) Ingenia 46:39-42; Bang et al. (2005) Proceedings of the 27th International Conference in Cement Microscopy; US Patent Publication No. 2011/0262640; U.S. Pat. No. 8,460,458.
Two different mechanisms for microbiological deposition of CaCO3 are known. The first is a urease based mechanism (see e.g., Van Tittelboom et al. (2010) Cement and Concrete Research 40: 157-166; Bang et al. (2005) Proceedings of the 27th International Conference in Cement Microscopy; US2011/0262640). The bacterial urease catalyzes the hydrolysis of urea into ammonium and carbonate. The carbonate ions produced react with cations from the environment, such as Ca2+, (e.g., from CaCl2.2H2O) which leads to the precipitation of CaCO3 at the cell surface. Several ureolytic bacteria of the genus Bacillus, which is a spore forming bacterium, are known to precipitate CaCO3.
An alternative metabolic mineral-producing pathway has also been used for deposition of CaCO3 (see e.g., Jonkers, H. M et al. (2010) Ecological Engineering 36: 230-235; U.S. Pat. No. 8,460,458). In this pathway the bacterial conversion of calcium lactate to yield CaCO3 and CO2 is exploited. The produced carbon dioxide further reacts with portlandite (Ca(OH)2) to form additional CaCO3 and water.
Another bacterial-based methodology for improving the compressive strength of a cement-based material uses bacteria to deposit films of organic materials (biofilms) which are capable of increasing the compressive strength of a cement-sand mortar (see e.g., Park et al. (2012) J. Microbiol. Biotechnol. 22(3): 385-389).